In fabricating an internal ring gear of a planetary gear train of a conventional gear reduction device, the internal ring gear is usually firstly fabricated, and mounting portions for connecting with a flange are then formed using a machining process. The internal ring gear fabricated in this manner requires secondary machining and thus has a high cost. In addition, eccentricity may easily occur during the fabrication or assembly process, which thus affects the concentricity between the internal ring gear and the flange once they are connected.